Fasteners are used in a wide variety of applications, such as mills, motors, railroad tracks, flange assemblies, petrochemical lines, foundations, drag lines, power turbines and studs on cranes and tractors. In many applications, achieving the proper fastener loading (tension) and maintaining this loading once the system is placed in service can be problematic. Monitoring the load on a fastener during installation and service can be equally problematic.
As a fastener is continually tightened, the load increases until the fastener ultimately yields, breaks, or its integrity is otherwise compromised. Thus, overtightening a fastener can lead to catastrophic failures. Conversely, fasteners typically experience some loss of tension in service due to, for example, a variety of in-service occurrences including: relaxation (thread embodiment), vibration loosening, compressive deformation in the joint or flange, temperature expansion or contraction, etc. Loss of tension from these occurrences can cause misalignment or premature wear in a bolted assembly, leakage (in applications where the fastener is used for sealing), or catastrophic joint failure due to excessively high loads on other members of the assembly.
In certain applications, knowledge of a fastener load, upon installation and over time, is desirable for avoiding the potentially dangerous consequences of a compromised or loosened fastener, such as slippage, wear, leakage and/or possible failure. In other applications, for example when working with a group of bolts around a flange of a sealed assembly, it is important to evenly tighten the group of bolts. By uniformly tightening a group of bolts or studs to an appropriate load, and maintaining this load over time, potential failures are less likely to be experienced.
Determining the tensile load of conventional fastener often entails cumbersome methods to check the tightness of each bolt, such as loosening and re-tighten all of the fasteners regardless of whether such re-tightening is needed. The retorquing (i.e. tightening) of a fastener, however, induces wear and strain in the fastener system. Additionally, corrosion, friction, variations in nut condition, and the like can cause variations in torque values and introduce error into tensile load measurements.
One earlier proposed load indicating fastener, which is the subject of U.S. Pat. No. 5,668,323 issued Sep. 16, 1997 to Cory S. Waxman and incorporated herein by reference, includes a single pivot lever positioned within a bore in the fastener head with the actuator end of the pivot lever in contact with a reference post or datum rod seated or formed in the end of the bore and an indicator end of the pivot lever being visible at the head of the fastener.
Other proposed methods often require costly tools or readers or special training. For example, electronic or ultrasonic methods for determining tensile loads require experienced operators, expensive equipment, clean surfaces and records of pre-installation test values for each bolt or stud. In addition, such systems may be adversely affected by shock and other extreme conditions.
Accordingly, improvements are sought in the monitoring of loading in fasteners.